Using a wooden spoon, beat egg yolks and two-thirds of the sugar until pale, then stir in flours and combine well. Add lemon juice, lemon rind,milk and melted butter and combine well. In a separate bowl, whisk egg whites until soft peaks form, gradually add remaining sugar and whisk until stiff peaks form. Fold egg whites into lemon mixture gently, in two batches, until just combined. Pour into greased dish and place in a deep roasting pan, then pour in enough boiling water to come halfway up side of dish and bake at 180C for 45 minutes or until puffed and golden.